@tauon i find matrix somewhat messy. it's decentralized, but there is an official registration server. and most registration servers use google recaptcha. the ones that don't usually get blocked to prevent spam.
signal is alright, but it's centralized and requires a mobile app to use. even if you just want to use it on your desktop computer.
irc is alright, but somewhat limited on its own when it comes to things like file sharing and encryption.
i also though about creating my own. but i don't know how to do any network related things.
for client software i'd have made a library in a low level language that contains everything a client needs to work, apart from an interface. this way people can easily create their own clients for various platforms.